Method for determining the carbon concentration in a metal melt

ABSTRACT

A method for determining the carbon concentration in a metal melt. The oxygen activity in the melt is first determined from a measurement of the temperature and the oxygen potential. The carbon concentration is then determined from the oxygen activity, for example with the aid of a suitable graph.

O United States Patent [151 3,661,725 Ulrich et al. [451 May 9, 1972 [54] METHOD FOR DETERMINING THE CARBON CONCENTRATION IN A Referencta Cited METAL MELT UNITED STATES PATENTS [72] Inventors: Klaus-Herbert Ulrich, Dusseldorf; Dieter 3 523 066 8 1970 M t 1 Doering EssemHolsterhausen; Kurt orns e a 204/l95 Borowski, Essen, all of Germany OTHER PUBLICATIONS [73] Assignee: Fried. Krupp Geselischaft mit Fitterer, Jour. ofMetals, Sept. I967, pp. 92-96.

beschrankter Haftung, Essen, Germany Primary E.\'aminerT. Tung [22] Flled' 1969 Attorney-Spencer and Kaye [211 App]. No.: 809,766

[57] ABSTRACT [30] Foreign Application Priority Data A method for determining the carbon concentration in a metal melt. The oxygen activity in the melt is first determined from 21 Mar. 22, 1968 Germany ..P 17 73 027.7 measurement of the temperature and the oxygen potentiaL The carbon concentration is then deten'nined from the oxygen [52] US. Cl ..204/l T activity f example with the aid f Suitable graph. [51] .....G01n 27/46 [58] Field of Search ..204/ l T, 195 S 1 Claims, 2 Drawing Figures PATENTEBMAY 9:912 3,661,725

SHEET 1 OF 2 lnvenkom;

moms-Herbert fluid .Ii. r Joeri PATENTEDHAY 9 m2 3. 6 61 ,7 2 5 SHEET 2 or 2 l M 1 1, a5 1,5 2 25 3,0 3,5 1 0 45 9% -%C F/G.3

Inventors:

Klaus-Herbert ULrLcb Dicker Joerin Kurt .iorousski fittornags METHOD FOR DETERMINING THE CARBON CONCENTRATION IN A METAL MELT BACKGROUND OF THE INVENTION pl'n. is the unknown oxidation potential (to be calculated); and

1, is the transport number of the ions.

The Nemst equation is stated by C. Wagner in the German The present invention relates to a method for determining 5 journal "zeilschl'ift fur Physikalische Chemie" P B 21 the carbon content of a metal melt.

The methods used in the prior art for determining the carbon concentration in molten metals have not been satisfactory for every situation. When employed during the refining process and, in particular, during conversion with air or with industrial grade pure oxygen, the test results have not been sufficiently exact. In addition, these prior art methods have unduly delayed the air or oxygen blasts, and thus have increased the time required for refining.

SUMMARY OF THE INVENTION An object of the present invention is to provide an improved method for determining the carbon concentration in a metal melt and, in particular, a method which is free of the above mentioned disadvantages of the concentration determining methods known in the prior art. V

This object, as well as other objects which will become apparent in the discussion that follows, is achieved, according to the present invention, by carrying out an electrochemical measurement of the oxygen potential of the melt and determining the oxygen activity from this measurement and from the temperature. By the relation between the oxygen and the carbon activity it is possible to obtain the latter from the former and thus, finally, determine the carbon concentration of the melt.

When refining a crude iron melt, for example, in an oxygen blown converter, the accompanying elements (metalloids), carbon, silicon, manganese and phosphorus, are more or less completely removed from the metal phase by oxidation. The carbon is converted to the gaseous phase while the silicon, manganese and phosphorus are collected as slag. The oxidation reactions take place not only in the so-called focus" at the gas/metal and slag/metal phase boundaries, but also within the metal melt. For this reason, the oxygen activity in the metal bath is of influence on the refining cycle. Dependent upon the temperature, the metalloid content of the melt, the composition and quantity of the slag as well as the blast conditions of the converter, a particular oxygen potential is established in the melt.

Since the metalloids, silicon and manganese, are for the most part scorified during the first minutes of the converting process and since, given the conventional concentrations, the influence of the phosphorus on the oxygen activity is practically negligible, the oxygen activity during the later stages of converting is almost exclusively dependent upon the carbon content and the temperature of the melt.

The determination of the oxygen activity is made by first obtaining the oxygen potential using a well-known electrochemical cell method which employs solid conductive electrolytes. It is possible, for example, to employ as the electrolyte ZrO which is doped with approximately 15 mole percent CaO. If the one electrode of the solid conductor cell is provided with a known or specified oxygen potential, e.g. air having an oxygen partial pressure of 0.21 mm, it is possible to calculate the unknown oxygen potential of the melt at the second electrode by measuring the electromotive force (emf) of the cell and using the Ne rn st equation:

In this equation:

z is the charge number of the charge carriers in the electrolyte; E is the electromotive force (emf); F is the Faraday constant (96,484 coulombs); 4];- is the specified oxidation potential;

(1933) page 25.

Given the oxygen potential plus the simultaneously determined temperature of the melt, the corresponding oxygen activity can then be determined by the following equation:

lna l/(R- T) (z'F- E+%AG)+% ln 0.21 (2) where :1 is the oxygen activity, R is the gas constant, T is the absolute temperature and AG is the free enthalpy of formation of the solution of 1 mole of oxygen in the corresponding metal or alloy.

This equation is quoted by C. Wagner and K. Kiukkola in the US-journal Joumal of the Electrochemical Society vol. 104 (1957) page 379 and by W.A. Fischer and W. Ackermann in the German journal Archiv fur das Eisenhuttenwesen" vol. 36 1965) page 643.

Statements relating to AG" are given by W.A. Fischer and W. Ackermann in the German journal "Archiv fur das Eisenhuttenwesen vol. 37 (1966) page 43.

The oxygen activity a can therefore be calculated from the measured values of the electromotive force E and the temperature T and from the values which, for example, in the case of iron are well known for the free enthalpy of formation AG".

Finally, since the oxygen activity for example, in steel melts with a carbon concentration of less than 1 percent exhibits a characteristic dependence upon the carbon concentration, it is possible to determine this concentration using suitable'calibration curves. These curves can be made initially by independent determination of the oxygen activity and the carbon content and then used to show this dependence for later measurements made according to the method of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTS The determination of the carbon concentration of a metal melt, such as steel, for example, may be made continuously during the refining process or made discontinuously at particular times during the process with the aid of a measuring lance". The last type of measurement will introduce no great delay in the refining process if the emf measurement is made simultaneously with the usual temperature measurement.

FIG. 1 schematically illustrates apparatus which may be used for the continuous determination of the carbon concentration. The converter is designated as 1, the oxygen lance as 2, the metal melt as 3 and the slag as 4. Built into the converter 1 is a thermoelement 5. This thermoelement is electrically connected with an external device 6 which measures the temperature of the melt.

The converter 1 is also provided with a solid conductive electrode 7 having a prescribed oxygen potential, e.g. that of air, as well as a second electrode 8, which can consist of an iron rod. The two electrodes 7 and 8 are connected to a device 9 which determines the electromotive force (emf).

FIGS. 2 and 3 illustrate the empirically determined dependence of carbon concentration upon oxygen activity for two types of melts. FIG: 2 illustrates the characteristic dependence for low phosphorus molten metal while FIG. 3 illustrates this dependence with high phosphorus molten metal. The data for both FIGS. 2 and 3 were taken from an oxygen blast converter.

The carbon concentration, determined by one of the conventional or prior art techniques, is plotted on the ordinate while the oxygen activity, calculated from the measured electromotive force and temperature, is indicated on the abscissa of each of the two graphs.

For a carbon concentration below 1 percent the characteristic oxygen activity carbon concentration dependence will be as shown for all other melts of the same type. Once this dependence has been determined, therefore, it is only necessary to calculate the oxygen activity from the measured values of electromotive force and temperature and read off the associated carbon concentration of the melt from the characteristic calibrated curve. So long as the carbon concentration is below 1 percent, the accuracy of measurement will suffice for all normal requirements.

it may be seen, therefore, that the method according to the present invention for determining the carbon concentration in a metal melt is essentially a two-step method. The oxygen activity is initially calculated, given the empirically determined electromotive force and temperature, using the equation 2 above. Given the oxygen activity, the carbon concentration is .then read from a suitable graph of the type illustrated in FIGS.

2 and 3.

Since the success of this method according to the present invention is dependent upon the correct determination of the oxygen activity, the derivation of the equation 2 will be summarized below.

Assuming that the transport number of the ions in the electrolyteis equal to l in the first approximation and that air having 0.21 atm. of oxygen is employed as the reference potential, the oxygen partial pressure of the melt 7 will be given by:

The solution of oxygen in metal melts takes place according to the reaction equation:

2 0 l The equilibrium constant of this reaction, K, is:

Thus, inserting equation 4 into equation 3 it is possible to calculate the oxygen activity a, in terms of the other variables and constants. A convenient form of the resulting relation is given as equation 2 above.

EXAMPLE at the bottom of the tube by sintering. This point also was for their carbon content in a convenient manner. At the same time the electromotive force between the iron rod electrode and the platinum wire was measured by a digital volt meter with an inner resistance of l l Megohms (Bee man Multime ter) and also the temperature of the melt was measured from which the oxygen activity was calculated. in this way every minute one point of the curve was obtained.

In the following data for one point of the calibration curve are given: Analyzed carbon content in the melt at the time of this measurement: 1.04 percent C;

measured temperature of the melt: 1,590 C;

measured electromotive force (emf): 1.075 Volts;

z number of charge of oxygen-ions O" 2;

F= Faraday constant 23,066 cal/Volt mole;

R gas constant 1.9865 cal/mole degree;

T= absolute temperature in Kelvin-degree l,863 K;

p 0.21 atm. In the following equation the natural logarithm 1n mentioned in the equation 2 above is replaced by the decade, or base 10 logarithm 1g which convention can be attained by the factor 1/2.303.

lg a

+% (86,580+ 15.32-1,863)]+% lg 0.211 a 2.75 17 It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations.

We claim:

1. A process which comprises:

a. blowing a melt of a carbon-containing metal with oxygen or air.

b. taking samples of the melt at intervals during blowing and without interrupting the blowing,

c. measuring oxygen potential and temperature of the melt simultaneously with the taking of each sample,

d. calculating oxygen activity for the melt from the oxygen potential and temperature and corresponding to each sample,

. analyzing each sample for carbon content,

preparing a plot of carbon concentration versus oxygen activity for the melt based on oxygen activity calculations from step d and carbon analyses from step e, and (g) refining a melt of a carbon-containing metal of the same type as the aforesaid carbon-containing metal melt by introducing an amount of air or oxygen into the latter melt based on the reading of carbon concentration from the plot. 

